Exciting means for high-frequency display tubes



2 Sheets-Sheet l far Fuller c. H. FULLER EXCITING MEANS FORHIGHFREQUENCY DISPLAY TUBE 7 Jan. 18, 1949.

Filed May 31, 1947 Jan. 18, 1949. c. H. FULLER 2,459,422

EXCITING MEANS FOR HIGH-FREQUENCY DISPLAY TUBE Filed May 51, 1947 2Sheets-Sheet 2 '77 75 7e f l 75 12 5. In V5.27 for 7- lfarveyj' u]! 51-Patented Jan. 18, 1949 EXCITING MEANS FOR HIGH-FREQUENCY DISPLAY TUBESCharles Harvey Fuller, Los Angeles, Calif.

Application May 31, 1947, Serial No. 751,545

8 Claims.

This invention has to do with improved means for exciting gas dischargetubes, particularly those which have no internal electrodes. Such tubesare commonly made in the form of letters or symbols and are assembled toform a luminous sign for advertising purposes, Although the presentinvention is particularly useful in such applications it can be usedalso for the excitation of other kinds of luminous discharge tubes, suchfor example as are sometimes formed in table glassware and the like. Forclarity of description the invention will be described as it has beenapplied to a particular type of luminous discharge sign, but I do notintend that the scope of the invention be limited to this application orrestricted to the particular structure which I have chosen for purposesof illustration.

It is well known that an electrical discharge tube, containing asuitable gas or mixture of gases at reduced pressure, can be excitedwithout the use of internal electrodes by placing it in a rapidlyalternating electric field of suiliciently high frequency and intensity.Such an alterating field is readily produced in the immediateneighborhood of the plates of a condenser which is connected in theoutput circuit of a suitable radio frequency oscillator. The condenserplates can, for example, be parallel rods or strips of conductingmaterial embedded in a sheet of dielectric material. Discharge tubesmade in the form of individual letters and the like can then be arrangedagainst the face of the sheet to form the desired message. If thenecessary conditions are met, gas within the tubes becomes ionized andsupports an oscillating electrical discharge which has the frequencycharacteristic of the oscillator but produces an apparently continuousluminosity in the tubes. The condenser plates will be referred to aselectrodes, since they perform in many respects the functions of theinternal electrodes of the more conventional discharge tubes. However,in the present system the electrodes are capacity coupled, rather thandirectly coupled, to the ionizable gas within the discharge tube.

Luminous signs of the type described have the advantage that theindividual letters can readily be replaced when necessary, and can bere-arranged to form any desired message. A disadvantage of such signshas been the difficulty of arranging the electrodes in such a Way thatthe letters of the sign can extend over a sufficiently large area andyet be excited to substantially uniform luminescence. A furtherdifficulty, which becomes particularly important if the voltageamplitude of the oscillating circuit is increased in order to insurestarting of the tubes and adequate excitation of all portions of theletters, is the tendency of the oscillating circuit to emit radiofrequency radiations of appreciable intensity and thus interfere withnormal radio reception in the vicinity of the sign. Partly for thisreason it is desirable, if not necessary, to design the electrode systemso that with minimum voltage and minimum power loss it will initiateionization reliably and then excite the letters uniformly.

A simple electrode arrangement, which is partially satisfactory forletters of relatively small size, say 2" in height, provides twoparallel electrodes, separated by approximately the height of theletters, and connected one to each output terminal of the source of highfrequency potential. When letters are placed in a row parallel to suchelectrodes and immediately in front of them, the vertical strokes of theletters extend generally along the lines of electric forces between thetwo electrodes. Once the discharge is initiated, the vertical letterstrokes are lighted with good uniformity, but the cross-strokes of theletters, and particularly intermediate cross-strokes as in the letter E,may remain dark. As the separation of the two electrodes is increased topermit the excitation of larger letters, the intensity of the resultingelectric field is correspondingly reduced, and the difficulty oflighting the cross-strokes tends to increase. This difliculty cannot beovercome merely by increasing the width of each electrode whilemaintaining a fixed space between them, because the electric field thentends to concentrate in the space between the electrodes, producingintense luminosity in the central portions of the letters, but leavingthe upper and lower extremities of the letters insufficiently excited.

Another arrangement which has been used to extend the efiective area ofexcitation of an electrode assembly employs a plurality of parallelelectrode elements, arranged in a common plane, alternate elements beingconnected together and to one terminal of the oscillator, and theintermediate elements being connected to the other terminal. This methodis successful in some respects, but a typical electrode of one polaritythen lies between two electrodes of the opposite polarity, whose fieldstend to cancel. Thus the portion of a letter which is directly oppositean electrode element is less brightly illuminated than the portionbetween electrodes, and mayeven be completely dark.

The present invention retains the basic simplicity of the two parallelelectrodes described above, but permits the successful starting anduniform illumination of larger letters than have previously beenavailable. My electrode system not only maintains full illumination inthe central portions of the vertical strokes, but also producessubstantially uniform discharge in the horizontal strokes, includingthose at an intermediate level. An important feature of the invention isthe addition of an auxiliary intermediate electrode element, generallyparallel to the upper and lower electrodes of certain previous systems,but so arranged that the electrical capacity between it and the ionizedgas in a typical discharge tube is less than the corresponding capacitybetween either of the main electrodes and the ionized gas. For example,the intermediate electrode may be spaced back of the plane of the mainelectrodes so that there is an increased air gap between it and thedischarge tubes, or the capacity may be limited by using an electrodeelement of small transverse dimensions. The intermediate electrode isconnected directly to one of the main electrodes, preferably to the onewhich is at ground potential. When such an intermediate electrode isused the separation of the two main electrodes can be considerablyincreased over previous practice, thus increasing the permissible letterheight.

A further advantage which is obtained with such an auxiliary electrodeis the more effective excitation of intermediate horizontal strokes inletters such as A, E, etc. For this purpose the intermediate electrodeis preferably placed directly back of such intermediate strokes. It thenserves to attract lines of force from the main electrode of oppositepolarity, producing a sufficiently strong field to light theintermediate cross-strokes; and yet, because of its limited capacity, itallows a sufficiently intense field to extend between the mainelectrodes to insure adequate excitation of the remainder of the letter.The use of an intermediate electrode not only increases the letterheight which can be successfully handled, but actually improves theuniformity of illumination, particularly in the intermediate horizontalstrokes of the letters. With given typical oscillating voltage supply,letters (made of 8 mm. glass tubing) as large as 6" in height aresuccessfully excited with my electrode system, as compared with amaximum practicable letter height of about 3" with previous systems.

Other objects and advantages of the invention will be clear from thefollowing detailed description of an illustrative preferred embodiment.This description is to ,be read in conjunction with appended drawings ofwhich Fig. 1 is a front elevation of a typical complete sign, accordingto my invention;

Fig. 2 is a section at enlarged scale, taken on the line 2--2 of Fig. 1;

Fig. 3 is a partial elevation showing a portion of the rear face of theelectrode supporting panel;

Fig. 4 is a partial horizontal section taken on the line 44 of Fig. 3;

Fig 5 is a schematic diagram of a typical electrical oscillator circuitsuitable for use with my invention; and

Fig. 6 is a perspective view of a preferred form of bracket foradjustably mounting individual discharge tubes.

Discharge tubes, shown in the form of typical letters 20, 2!, areremovably mounted, by means to be described, in a row 22 across thefront face of a panel 25 of dielectric material, such for example as thepolymerized phenolic resin known commercially as Bakelite. Panel 25conveniently forms the front wall of a case lit, the other sides ofwhich are of metal, and within which is mounted the radio frequencyoscillator 35, comprising transformer 35, rectifying tube 3?, oscillatortube 38, and associated circuitry, contained in chassis 39. Aconventional oscillator circuit such as that shown schematically in Fig.5 can be used, and is preferably capable of producing an oscillatingcurrent of the order of 6 amperes at any convenient frequency above,say, 75,000 cycles per second, and preferably in the diathermy band atapproximately 31 megacycles. A direct current plate voltage of about 300volts is generally sufficient for oscillator tube 33, leading to a peakvoltage in the oscillating circuit in the neighborhood of 500 volts. Theelectrodes (or electrode assemblies) by means of which discharge tubes2ll, .2! are excited are connected respectively to the live outputterminal of the oscillator, shown at ill, and to ground, which includesthe metal case and oscillator chassis The capacity shown at $9 in theoutput circuit of the oscillator is substantially the capacity betweenthe tube exciting electrode systems when no discharge tubes are excited,and is substantially the series combination of the separate capacitiesbetween the respective electrode systems and the ionized gas in thetubes when the latter are excited. Thus the load capacity of theoscillator varies widely with the conditions of use.

"The live electrode in the modification illustrated comprises a singlestrip 50 of conducting material, preferably copper foil. As shown, thisis embedded in panel 25, parallel to and just behind the front panelface 28, and at the general level of the lower edge of the row ofletters 22, adjacent the lower horizontal strokes of the letters, showntypically at H2, H2a. Lower electrode 5c is electrically connected bylead 55 to live terminal of the oscillator. The grounded electrodesystem includes upper electrode 5!, embedded near the front face ofpanel 25 at the general level of the top edge of the letters 22, andadjacent upper horizontal strokes, such as l l3, Ilia; the intermediateelectrode 52, which is mounted on the rear face of the panel; and alsothe letter supporting rail 86 and clips Hill, which are more fullydescribed below.

Intermediate electrode 52 is roughly half way between electrodes 58 and5|, and is approxi mately at the level of the intermediate horizontalstrokes of letters 22. Such strokes are shown typically at H 4, H40, inFig. 1. Electrodes 5i and 52 are electrically connected. to each otherby leads 5S and to any convenient ground, such as the case oftransformer 36, by lead 51.

A convenient manner of mounting the electrodes in the positionsdescribed is shown clearly in Figs. 3 and 4. Panel 25 is composed of twolayers, main panel in, providing the principal structural strength, andpanel facing ll which acts as an insulating layer across the front ofthe panel, and provides a uniform background against which letters 22can be viewed. In Figs; 2 and 4 the thickness of the panel layers andalso that of the electrodes is exaggerated for the sake of clarity ofillustration. Electrodes and 5i are threaded through slots l5 cut inmain panel Ill, and are also glued or cemented in place. Extra foilsections 59 are inserted to provide a substantially continuous foilsurface on the front of the panel. Panel facing ll is then glued orcemented to the front of main panel iii, covering the electrodes andbinding them firmly in place. Intermediate electrode 52 is mounted onthe rear face of main panel "ill, so that it is spaced back from theplane of electrodes 5E and ill by the thickness of this panel layer. Theposition of the electrode on the panel is definitely determined bysuspending it from upper electrode 5! by its ends as shown. at '56, andby one or more straps as indicated at 11, the straps serving also asconnecting leads. Electrode 52 may be further securedto the panel byglue or cement.

I preferably use relatively thin dielectric material, say /64 thick, forpanel facing ll, so that the upper and lower electrodes aresubstantially in the front face 28 of the combined panel 25, and yet areelectrically insulated from any ob jects which may come in contact withthat face. The thickness of the main panel member 1!! can be varied tosuit requirements, and in particular is a convenient means ofautomatically determining the spacing between intermediate electrode 52and the plane of electrodes Eli, 5i. In general a panel thickness ofabout 3 is found to give satisfactory results under typical conditions.

The fact that electrode 52! is set back from the plane of electrodes 59and El results in a lower electrical capacity between electrode 52 andthe ionized gas in a typical discharge tube than the capacity betweenelectrode 51 and the discharge tube. Because of its lower capacity,electrode 52 does not draw tube current to the exclusion of electrode5i, and hence does not prevent the upper portion of a tube from beingproperly excited. If electrode 52 is made narrower the capacity isreduced accordingly, and it can then be positioned closer to the planeof electrodes 56 and 55. It can even be placed in that plane if itstransverse dimensions are sufliciently reduced, as when a wire insteadof a strip of foil is used. However, the arrangement illustrated. ispreferred to that just suggested, since the latter tends to producelocalized excitation of the tube, and hence to be less flexible in usethan the preferred form.

Since electrodes 5| and 5.12 are directly connected together (by leads55) they function in some respects as a single electrode unit, and maybe constructed as an integral unit if desired. However, for mostpurposes it is preferred to use a structure of the general typeillustrated, in which electrode elements 5! and 52 are separateconductors vertically spaced from each other. This gives a smaller totalelectrode area, tending to reduce the dielectric power loss, and yetconcentrates the electric fieldwhere it is most needed, namely in theneighborhood of the horizontal letter strokes.

It will be understood that the intermediate electrode may be connectedif preferred to live electrode 55 rather than to grounded electrode 5!,and that such an arrangement will function in many respects like thearrangement illustrated, particularly in giving improved excitation ofintermediate letter strokes such as ti l, this. However, it is generallyadvantageous to hold the surface of the live electrode system to aminimum to reduce radiation and power loss.

The length of panel 25 is largely arbitrary, depending upon therequirements of the user. The preferred method of mounting theelectrodes in panel 25 permits them to be extended to any lengthrequired, supporting slots 15 being introduced at any desired intervals.In general the power drawn from oscillator 35 increases with the numberof letters excited, so that in practice the electrode length that can besatisfactorily employed with a given oscillator is limited. Furtherextension of the sign can be accomplished, for example, by constructingthe electrodes in sections insulated from each other, and powering eachsection from a separate oscillator.

The electrode system as so far described is ad-- vantageous quite apartfrom the particular meth od used for mounting the discharge tubes 22against the front face of panel 25. However, the type of tube mountingillustrated is particularly effective and convenient in its mechanicalaction, and also cooperates electrically with electrodes 50, 5| and 52to give improved uniformity of discharge during normal operation andespecially to facilitate initiation of the discharge at the start ofoperation. Certain features of this mounting system are described andclaimed in my copending patent application, Serial No. 621,432, filedOctober 10, 1945.

In the preferred embodiment illustrated, a metal mounting rail allparallel to and spaced forwardly from panel 25 is formed integrally withcase 30. The bottom 88 of the case extends forward and is bent upward toform first the vertical flange portion 87 and then the curved flangeportion 83. Flange 88 carries an inwardly facing shallow channelformation 89 near its upper edge. The entire rail formation 88 is atground potential, being electrically connected to metal case 30, andpreferably integral therewith.

Angle strip 95 is secured to case bottom 86, as by spot-Welding, andforms with flange 81 a channel 98 which receives and supports the loweredge of panel 25, insuring a definite spaced relation between the paneland groove of the rail formation.

Each discharge tube carries a downwardl extending stub H0, which may bean extension of a vertical letter stroke 5 i l, as in letter 20 of Fig.1, or may, for example, be connected approximately at the midpoint of alower horizontal let ter stroke '2, as in letter 2i. The stub H0 ispreferably formed integrally with the main body of the letter, andcontains a hollow chamber which is an extension of the main gas filleddischarge chamber of the letter.

A special mounting bracket 5% has a resilient clip portion Hll adaptedto at least partially enclose and grip the stub ill? of a dischargetube; and rail engaging portions H35 which are so formed as to seat ingroove '89 of rail 81?. The portions I05 are resiliently related to clipportion ill! in such a way that when a bracket is mounted on a letter asshown in Fig. 6, portions Hi5 have a relatively free resilient motion ina direction normal to the plane of the letter, but are relatively stiffagainst motion parallel to that plane, at least in a vertical direction.Such action can be obtained, for example, by mounting portions ingdownward from each arm Hill near the angle of the L is an ear illl,adapted to contact the lower part of curved flange 88, facilitating theplacement of the letters by limiting the downward movement of theletter, and protecting the stubs i I during the placement.

When the letters are mounted on the sign as in Fig. Z,.portions H35 ofbrackets Hill, resiliently urged into engagement with channel 89 of railfill, act as spring detents to define the rotational and verticalposition of the letter in its plane. The spring action of arms W alsopresses the lower portion of the latter into contact, with panel 25 at apoint below the level of channel 39, say at point H5 in Fig. 2. Thelatter contact in combination with the rail contacts M5 exerts arotational couple upon the discharge tube as a whole, in acounterclockwise sense as seen in Fig. 2, tending to hold the upperportionof the letter closely against the front face 28 of panel 25. Thelongitudinal position of the letter along the length of the sign isdetermined only by friction against panel 25 and rail channel 89, and isreadily adjustable manually. An advantage of this method of mounting isthat such longitudinal adjustment of the letters along the row 22 can bemade with perfect safety while the letters are illuminated, The detentaction already described prevents accidental displacement of theletters, and yet permits them to be readily lifted ofi the sign orpressed down into position on it without the need of first turning offthe electrical power.

In addition to providing a convenient mechanical mounting for theletters, brackets I06, contacting the grounded rail 83, provide agrounded electrode which in direct contact with the stubs I it) of thedischarge tubes.

Because of this direct contact there is a relatively intense electricfield between live electrode 56] and clip Hill when the power is firstturned on. This ionizes gas in the tube, initiating the discharge, whichthen spreads to the remainder of the tube. Thus the bracket insuresreliable starting of the sign even when the mutual separation ofelectrodes 50, El and 52 is relatively large. But because of therelatively small area of the portion of clip iill which contacts thetube, the capacity between the clip and the ionized gas in the tube isnot very great and hence does not draw excessive current duringoperation.

During normal operation, the field between live electrode as andgrounded rail 89 (particularly the upper edge of this rail) passestransversely through the lower horizontal strokes N2 of the letters,tending to maintain a strong discharge in these portions, which mayotherwise be difiicult to excite. Use of a grounded rail 80 thus permitselectrode 56 to be at a higher level with respect to the letters thanwould otherwise be practicable. On the other hand, the rail does notdraw tube current to the exclusion of grounded electrodes 51 and 52,since the rail proper is spaced from the surface of the letters, andsince the total area of clips lili which contact the letters isrelatively small.

Regarding the terminology used in the present specification and claims,directional terms such as vertical, horizontal, upper, lower, and thelike, refer to directions relative to the letters of the sign as a signis normally viewed, and not necessarily relative to gravity. Terms suchas the plane of the panel are not intended to imply that the panel, orthe letters associated with it, must necessarily be fiat, On thecontrary, panel 25 may be curved either cylindrically or spherically,but in practice such curvature is small enough so that any small regionof the panel defines a plane with reasonable definiteness. Similarly, ifthe electrodes are somewhat curved in the plane of the panel, mycharacterization of them as generally parallel will be clearlyunderstandable if any relatively short section be considered.

I claim:

1. Means for exciting a gaseous discharge tube of substantially planeconfiguration, said means comprising means for positioning the tube in agiven plane, two electrodes spaced. from each other and having arelatively high electrical capacity relation with respect to ionized gasin thepositioned tube, a'third electrode intermediate the said twoelectrodes and having a relatively low electrical capacity relation withrespect to ionized gas in the positioned tube, the third electrode beingelectrically connected to one of the said two electrodes, and electricalcircuit means for applying a high frequency oscillating voltage betweenthe other of the said two electrodes and the pair of connectedelectrodes.

2. Means for exciting a gaseous discharge tube of substantially planeconfiguration, said means comprising means for positioning the tube in agiven plane, two electrodes spaced from each other and each having arelatively large surface in proximity to the positioned tube, a thirdelectrode intermediate the two said electrodes and having a relativelysmall surface in proximity to the positioned tube, the third electrodebeing electrically connected to one of the two said electrodes, andelectrical circuit means for applying a high frequency oscillatingvoltage between the other of the said two electrodes and the pair ofconnected electrodes.

3. Means for exciting gaseous discharge tubes of substantially planeconfiguration some of which have a horizontal edge portion and anintermediate horizontal portion, said means comprising means formounting the discharge tubes in a horizontal row in a vertical plane, anelectrode unit having a horizontal limiting edge at the general levelof, but spaced from, the intermediate horizontal portions of thepositioned tubes and having a portion closely adjacent the horizontaledge portion of the positioned tubes, an electrode adjacent the plane ofthe positioned tubes and vertically spaced from the said electrode unit,and electrical circuit means for applying a high frequency oscillatingvoltage between the electrode and the electrode unit.

4. Means for exciting a plurality of gaseous discharge tubes in the formof letters and symbols having a definite maximum height, said meanscomprising two elongated generally parallel electrical conductors spacedfrom each other bya distance less than the said height of the letters, athird elongated conductor generally parallel to and intermediate thesaid two conductors and spaced from them and from their plane, the thirdconductor being electrically connected to one of the said twoconductors, means for removably positioning the discharge tubes in aplane generally parallel to the plane of the said two conductors and onthe opposite side of that plane from the third conductor, and electricalcircuit means for applying a high frequency oscillating voltage betweenthe other one of the said two conductors and the pair of connectedconductors.

5. Means for exciting a gaseous discharge tube, comprising a panel ofdielectric material having a front face, means for positioning adischarge tube in front of said panel face, two elongated generallyparallel conductors substantially in the plane of the said panel faceand transversely spaced from each other, a third elongated conductorgenerally parallel to and between the said two conductors and spacedback of the said panel face, the third conductor being electricallyconnected to one of the said two conductors, and electrical circuitmeans for applying a high frequency oscillating voltage between theother one of the said tWo conductors and the pair of connectedconductors.

6. Means for exciting a plurality of gaseous discharge tubes some ofwhich have upper and lower horizontal portions and intermediatehorizontal portions, said means comprising a panel of dielectricmaterial, means for removably positioning the discharge tubes adjacentthe front face of said panel in a horizontal row, a first elongatedconductor substantially in the plane of the said panel face at thegeneral level of the lower horizontal portions of the positioneddischarge tubes, a second elongated conductor substantially in the planeof the said panel face and at the general level of the upper horizontalportions of the positioned discharge tubes, a third elongated conductorelectrically connected to the second conductor and parallel to butbehind the plane of the first and second conductors at the general levelof the intermediate horizontal portions of the positioned dischargetubes, and electrical circuit means for causing the voltage of the firstconductor to oscillate at high frequency with respect to the voltage ofthe second and third conductors.

7. Means for exciting a gaseous discharge tube of substantially planeconfiguration, said means comprising a panel of dielectric material, anelongated live electrode embedded near the front face of the panel, anelectrically grounded metal rail spaced from the said face of the panel,the upper edge of the rail being substantially opposite the said liveelectrode, two grounded elongated electrodes generally parallel to thesaid live electrode and spaced above it, one of the said groundedelectrodes being spaced relatively farther than the other behind thesaid panel face, means for mounting the discharge tube in a planebetween the said panel face and the rail, and electrical circuit meansfor causing the voltage of the said live electrode to oscillate at radiofrequency about ground potential.

8. Means for exciting a gaseous discharge tube as specified in claim 7,and in which the said means for mounting the discharge tube include ahorizontal groove in the rail facing toward the panel and a bracketcomprising a resilient metal clip adapted to receive a verticallyelongated portion of the tube and two metal surfaces resiliently relatedto and electrically connected with the clip and adapted to engage thegroove at longitudinally spaced points thereof.

C. HARVEY FULLER.

No references cited.

